The present invention relates to a load-cell balance using a strain gauge.
The typical conventional load-cell balance of the kind described has a stationary portion fixed to a base, a movable portion to which a load support is connected and a pair of arms connected between the stationary and movable portions, the stationary portion, movable portion and two arms in combination forming a quadrilateral beam. The balance further has four strain gauges adhered to thin-walled flexible portions formed at both ends of each arm. These four strain gauges are connected to form a bridge circuit such that an electric signal corresponding to the load is derived from the bridge circuit.
Needless to say, in the load cell balance of the type described, it is necessary to process the beam as the mechanical constituent with high precision. In addition, since the balance involves electrical connection, the stability of the electrical connection is an essential factor.
The positions of the strain gauges are naturally determined, and the connection has to be made independently to each strain gauge by means of a lead. In the conventional load-cell balance, it is not possible to definitely fix the position of a lead, because the leads are derived independently from a circuit provided on the stationary part. In addition, electrical interference between the leads is inevitably caused, resulting in an unstable measurement. Further, the characteristic of the balance is changed by external force. The measurement is unstabilized also by the change in temperature of the strain gauge attributable to the ambient air flow. The lead wires exposed to the outside are likely to be damaged or cut. Moreover, the greatest care is required in handling of the balance so as not to damage the strain gauge adhered to the flexible portions of the arms and exposed to the outside of the beam.
The characteristic of the strain gauge is largely affected also by humidity, so that measures for protecting the gauge against the humidity are indispensable.
The desired effect cannot be achieved solely by connecting the strain gauges in the form of a bridge. Namely, it is necessary to connect other electric parts such as compensation resistors and the like. The connection has to be made in the circuit part resulting in a difficulty in adjustment.